There have been previously developed various techniques for obtaining measurements of formation conductivity with a relatively high resolution; i.e., with the ability to resolve conductivity, or changes in conductivity, over small distance variations in the formations. Most of these techniques are implemented with logging devices having pad-mounted electrodes or antennas. A review of existing techniques is set forth in copending U.S. patent application Ser. Nos. 616,323, now U.S. Pat. No. 4,712,070, 616,325, 616,326, and 616,327, all filed May 31, 1984, assigned to the same assignee as the present application, and hereinafter collectively referred to as the "copending prior applications". Some of these techniques, and the devices which employ them, will now be reviewed.
A valuable aid in the exploration for oil and gas is the dipmeter log, which provides positive structural and stratigraphic information for both exploration and development drilling programs. Advances in dipmeter tool design, machine computerization, and interpretation methods make it possible to recognize such features as structural dip, faults, unconformities, bars, channels, and reefs. In addition, the direction of sedimentation and of pinchouts can be estimated. When combined with data from other wells, dipmeter information helps to establish the overall structural and stratigraphic picture of the area under study.
The focused current type of dipmeter has been particularly well received by the wireline logging industry for use in logging boreholes drilled with conductive drilling fluids. Focused current dipmeter tools employ at least three pads and commonly four, each of which comprises one or more electrodes for emitting a focused current beam into the adjacent formation. The current flow at each electrode is proportional to the conductivity of the adjacent formation. Focused current dipmeters are described in U.S. Pat. No. 3,060,373 issued Oct. 23, 1962 to Doll; U.S. Pat. No. 4,251,773, issued Feb. 17, 1981 to Cailliau, et al.; and U.S. Pat. No. 4,344,271 issued June 8, 1982 to Clavier. These are able to achieve good vertical resolution at reasonable logging speeds, the microresistivity sensors used on some of these tools being capable of resolution to as fine as 0.2 inch.
The great amount of data acquired by dipmeters, and especially the high resolution focused current dipmeters, is advantageously exploited by the use of computers. For example, suitable computer implemented correlation techniques are described in U.S. Pat. No. 4,348,748, issued Sept. 7, 1982 to Clavier, et al., and U.S. Pat. No. 4,355,357, issued Oct. 19, 1982 to Chan. Improved dip determinations often can be obtained by use of other computer-implemented techniques, such as that described in U.S. patent application Ser. No. 383,159, filed May 28, 1982, now U.S. Pat. No. 4,453,219.
Electrical dipmeters, including the focused current type, are not altogether satisfactory for use in boreholes which have been drilled with a nonconductive fluid such as air or an oil-based mud. Techniques based on the principle of induction logging have been proposed for measuring dip by the use of either mandrel-mounted coils or pad-mounted coils. In conventional induction logging, such as disclosed in U.S. Pat. No. 2,582,314, issued Jan. 15, 1952 to Doll, oscillating magnetic fields formed by one or more energized induction coils on a mandrel induce currents in the formation around the borehole. These currents in turn contribute to a voltage induced in one or more receiver coils through a secondary magnetic field. The voltage component of the received signal that is in phase with respect to the transmitter current, known as the R-signal, is approximately proportional to formation conductivity.
Induction techniques using pad-mounted coils have been described, for example, in U.S. Pat. Nos. 3,388,323, 3,539,911, and 4,019,126. These techniques suffer various disadvantages, as described in the referenced copending prior applications.
In the referenced copending prior applications there are disclosed pad-mounted microinduction antenna arrangements for obtaining improved measurements in, for example, dipmeter logging devices and R.sub.xo logging devices. The antennas disclosed therein include loops and half-loops in various arrangements. Circuitry for obtaining output signals from the receivers, generally representative of conductivity or conductivity contrasts in the formations, are also disclosed. In an embodiment set forth in the prior applications, the receivers are connected in series opposition through a subtractive network such as a balun (a "balanced/unbalanced transformer"), which enables the direct mutual inductance to be nulled. The output of the subtractive network is coupled to an input of a phasesensitive detector, the reference input to which is a reference signal obtained from the transmitter. The reference signal is used as phase reference in determining the conductivity-induced component in the receivers, and in rejecting nuisance signals including the X-signal.
It is among the objects of this invention to provide improvements in microinduction antennas and logging systems. It is also among the objects of the invention to provide an improved apparatus for obtaining a reference signal for use in obtaining the conductivity-induced signals in the receivers of microinduction and induction logging systems.